1. Field of the Invention
The present invention relates generally to energy scavengers and energy harvesters which convert ambient forms of energy into electricity, and more particularly to vibrational energy harvesters that convert environmental mechanical vibrational energy to electrical energy utilizing piezoelectric type materials.
2. Description of the Related Art
One of the most familiar types of energy harvester is the solar cell. Additionally, there are energy harvesters that convert thermal gradients, wind power, water power, and the like into electric power. To take advantage of these types of energy harvesters, environmental contact such as exposure to light, thermal gradients wind, water, and the like is required. These types of energy harvesters are not commonly found in enclosed environmental spaces such as HVAC systems or walls, embedded in machinery or tissue, or other applications that do not have access to external environmental energy sources.
Alternatively, vibrational type energy harvesters (VEHs) are able to take advantage of environmental vibrations created by appliances, HVAC systems, equipment, motors, human movement, and the like, to produce electrical power or electricity in open as well as enclosed environmental spaces. These environmental vibrational frequencies are typically found in the 1-1000 Hz range. In general, a VEH comprises a proof mass on a spring. The spring is typically a cantilever beam. The mass/spring combination has a resonant frequency and, although it can respond to a spectrum of vibrational frequencies, it responds most strongly to ambient vibrations around that resonant frequency. This resonant motion can be converted to electricity using techniques such as electromagnetic pick-up or piezoelectric generation.
Energy harvesters are well known in the art for use in a variety of applications. However, due to the limitations described herein, they are not well known for wireless applications. They have been suggested for wireless sensor applications to provide the electrical energy to power a sensor and/or charge an associated battery. Wireless sensors measure environmental variables and transmit the measured data back to a receiver without any cabling or wires. Typically, the data is transmitted using radio frequency signals and the power for the sensor is provided by a battery, capacitor, or similar energy storage device. Wireless sensors have many potential applications because they can be used where it is difficult to get wires or as after-market system upgrades requiring no additional cabling. Some of the many applications that have been identified that could benefit from wireless sensors include:                Monitoring of infrastructure integrity, such as buildings and bridges,        Air quality monitoring in HVAC (heating, ventilation, and air conditioning) systems,        Monitoring industrial processes, such as chemical or food production,        Equipment health monitoring,        Medical implants or mobile personal health monitoring,        Automotive sensors, such as tire pressure monitors, and        Defense and security applications such as chemical and biological monitoring of buildings and public spaces.These applications have not developed as quickly as predicted, in part, because of the difficulty of providing power to the sensors. Although batteries can be used, they have a limited life and must be replaced periodically. Since remote sensors are typically placed in inaccessible places (because of the difficulty of running wiring), battery changing can be difficult or impossible. There is additionally a cost for using batteries which includes labor, the recurring cost of the battery, and disposal, with its attendant environmental concerns. One solution to this wireless sensor power problem is to provide power using energy scavengers or energy harvesters which can convert ambient forms of energy into electricity for use by the sensor or for charging the battery. Microfabricated MEMS VEH devices may provide such a solution.        
Microfabricated VEH Devices (μVEHs), however, have their own set of problems. Typically, a piezoelectric MEMS μVEH consists of a cantilever beam element capped with a piezoelectric film and terminated with a proof mass. When the device vibrates, the mass stresses the cantilever beam, including the piezoelectric film. The stress in the piezoelectric film generates charge and a voltage difference is created across the capacitor formed by the piezoelectric between the top and bottom surfaces.
The resonant frequency (f) of a spring/mass combination is represented by the equation f=(1/2π)(k/M)1/2, where k is the spring constant and M is the proof mass. Because of their small size (i.e. short springs and small masses), μVEHs typically have, high resonant frequencies (>500 Hz). In order to achieve the desired low resonant frequency configuration and sensitivity, the cantilever beam (spring) must be long (a few mm), thin, and compliant, and the mass must be large (a few milligrams). For MEMS μVEHs, this is difficult. MEMS fabrication is based on thin film processing techniques developed by the semiconductor industry. Conducting, insulating, semiconductor, and piezoelectric films are deposited using sputtering, vacuum evaporation, or chemical vapor deposition and are typically only 1-3 microns thick (or less). A 5 micron thick film is a very thick film deposition for a microfabrication process. It is relatively difficult to construct large structures with such thin film fabrication techniques.
Additionally piezoelectric μVEHs produce voltages of only a few hundredths of a volt and power levels of only a few μW or less. One of the primary reasons for such low outputs is the limited stress that can be applied to the piezoelectric element. It is known in the art that the stress in a bending cantilever beam element is proportional to the distance of the element from the neutral axis of the cantilever. The neutral axis is defined as the line where the stress is zero. When the cantilever bends downward, the stress above the neutral axis is tensile. Below the neutral axis, it is compressive. When it bends upward, the stresses reverse. If the neutral axis falls inside the piezoelectric material, the charge generated above the neutral axis is canceled by charge of the opposite polarity generated below. In the design of an energy harvester, then, it is desirable to keep the neutral axis outside the piezoelectric element.
The easiest way to get the entire piezoelectric film away from the neutral axis is to deposit it on a base. This is commonly achieved by placing the thin piezoelectric film on a thicker silicon (Si) or silicon dioxide mechanical cantilever. This single element, or monomorph, architecture can be fabricated several ways. One way is to deposit the mechanical base layer on a standard Si wafer prior to deposition of the piezoelectric and electrode films. This base layer must be relatively thick (in microelectronics terms), for example, 3-5 microns thick. Even so, the neutral axis will be just inside or just below the piezoelectric. A second approach is to use a silicon-on-insulator (SOI) wafer that has a single crystal silicon layer a few microns thick atop a buried oxide layer which again is atop a thick handle.
In both cases, the bulk of the silicon wafer is etched off the back under the piezoelectric, leaving the piezoelectric film and the support layer (either oxide or silicon) freely suspended. Because the neutral axis is so close to the piezoelectric element, only low stresses are produced, so these MEMS μVEHs tend to generate only a few hundredths to tenths of a volt per “g” of acceleration, where “g” is the acceleration due to gravity.
A second limitation of the MEMS monomorph architecture is that the mechanical energy that drives the cantilever must be shared between bending the support cantilever and bending the piezoelectric element. The energy that is required to bend the support does not go into straining the piezoelectric, thereby limiting the amount of voltage that can be developed.
One approach that has been employed to compensate for the low stress and enhance the output voltage is to connect multiple cantilevers in series. The difficulty with this approach is that in order to respond identically to input acceleration, the cantilevers and proof masses must be identical. Otherwise, they will have different resonant frequencies or phases and interfere with one another. Microfabrication process variations have to be well controlled. Additionally, valuable chip space is lost since it must be used for redundant cantilevers to boost output at a single frequency. This chip space could more advantageously be used for cantilevers with different resonant frequencies to broaden the band of harvested frequencies.
Another approach that has been taken to enhance the generated voltage is to fabricate a piezoelectric bimorph on the support cantilever. This, however, has its own difficulties. Fabrication of a MEMS bimorph requires several additional thin film depositions to prevent shorting of the layers together when bondpad metallization is deposited. These additional thin film depositions include the extra piezoelectric element, electrode and additional insulation layers. Several additional photomasks are also required to permit etching of the first electrode and piezoelectric element in order to gain access to the center electrode and to open up electrical, contacts in the insulating layer. One thin film piezoelectric, PZT (Lead Zirconate Titanate), is commonly used because of its high piezoelectric constant. However, PZT is very difficult to etch. Aluminum nitride (AlN) has been commonly used instead because it is very compatible with semiconductor processes, and can be etched. However, finding etches that are selective between MN and its common electrode material molybdenum (Mo) is also challenging. Another challenge is that precision is required when building the bimorph to get the numerous film thicknesses correct in order to place the neutral axis correctly.
An issue all the aforementioned MEMS approaches have in common is film stress gradient control. Each of the depositions are typically performed at different temperatures, such that as the film stacks are built up, differential thermal expansion of the substrate and films build up stresses in the stack. These are typically very tensile stresses. When the final release of the cantilever takes place, the stresses can cause the cantilevers to curl up, sometimes well over 360°. This built in stress can be mitigated by adding a compressive overlayer of oxide. Unfortunately, however, the thickness of this compensating layer has to be fine tuned to balance out the stack stress. This can be accomplished, but balancing acts are hard to maintain, as processes and materials possess variability.
It is therefore an object of the present invention to provide an energy harvester with a cantilever structure not prone to physical defects. It is another object of the present invention to provide an energy harvester with improved power output. It is yet another object of the present invention to provide an energy harvester with improved efficiencies and greater frequency range. It is another object of the present invention to provide an energy harvester with improved energy transfer. These and other objects of the present invention are not to be considered comprehensive, or exhaustive, but rather; exemplary of objects that may be ascertained after reading this specification with the accompanying drawings and claims.